This invention relates to a method for obtaining crystalline semiconductors for use in thin film devices such as thin film insulator gate type field effect transistors (Thin Film Transistors, or TFTs).
Conventionally, crystalline silicon semiconductor thin films used in thin film devices such as thin film insulator gate type field effect transistors (TFTs) have been manufactured by forming an amorphous silicon film on an insulating surface such as an insulator substrate by plasma CVD or thermal CVD and then crystallizing this film in an electric furnace or the like at a temperature of above 600° C. over a long period of twelve hours or more. In order to obtain particularly good performance (high field effect mobility and high reliability), heat treatment for even longer periods has been required.
However, there have been numerous problems associated with this kind of conventional method. One problem has been that throughput is low and therefore costs are high. For example, if 24 hours are required for this crystallization process, and if the treatment time for each substrate is 2 minutes, it has been necessary to treat 720 substrates at the same time. However, for example, in a commonly used tube furnace the number of substrates that can be treated at one time is 50 at the most, and when one only apparatus (reaction tube) is used the time taken per substrate has been as long as 30 minutes. In other words, in order to make the treatment time per substrate 2 minutes, it has been necessary to use as many as 15 reaction tubes. This has meant that the scale of the required capital investment has been great and that the depreciation on that investment has been large and has kept the cost of the product high.
Another problem has been the temperature of the heat treatment. Substrates commonly used in the manufacture of TFTs can be generally divided into those which consist of pure silicon oxide, like quartz glass, and no-alkali borosilicate glass types, like Corning Co.'s No. 7059 (hereinafter referred to as Corning 7059). Of these two classes, in the case of the former, because they have excellent resistance to heat and can be handled in the same way that substrates are handled in ordinary semiconductor integrated circuit wafer processes, there are no problems relating to heat. However, they are expensive, and their cost rapidly increases exponentially along with increases in surface area. Therefore, at present, they are only being used in TFT integrated circuits of relatively small surface area.
No-alkali glass, on the other hand, is of satisfactorily low cost compared to quartz; however, its resistance to heat is a problem, and because its distortion point is generally about 550 to 650° C., and in the case of particularly easily acquired materials is below 600° C., with heat treatment at 600° C. problems of irreversible shrinkage and warping have arisen. These problems have been especially conspicuous with large substrates of over 10 inches in diagonal. For reasons like these it has been considered in connection with the crystallization of silicon semiconductor films that heat treatment conditions of below 550° C. and less than 4 hours are indispensable to reductions in cost. An object of this invention is to provide a method for manufacturing a semiconductor which clears these conditions and a method for manufacturing a semiconductor device in which such a semiconductor is used.